Although a variety of ventilatory control studies have been performed in developing mammals, the maturation of motor responses in expiratory muscles has received little experimental consideration. However, the muscular assistance of expiratory airflow can be an important adaptation to the mechanical loading of expiration; and it is the aim of the proposed studies to consider how the maturing animal utilizes this control mechanism. The opossum (Didelphis Virginiana), a marsupial mammal will be our experimental model. As with all marsupials, the opossum is born at a very primitive stage of development. The limits imposed upon respiratory regulations due to immaturity can be evaluated in this preparation, and subsequent development can be closely followed. Some preliminary observations in the young opossum have shown that lung inflation and hyperventilation caused expiratory muscles to be activated, but only in animals over 40 days of age. Such expiratory responses in the opossum and other mammals depend upon vagally-mediated afferent activity; and it can be postulated, therefore, that the onset of expiratory motor activity in the opossum requires an appropriate maturation of vagal afferent fibers. On the other hand, the opossum shows evidence of vagally mediated pulmonary inflation and deflation reflexes, much before the onset of muscle activity in phase with expiration. From this information it can be suggested that the development of expiratory responses involves the maturation of other elements of the nervous system. To differentiate between vagal and non-vagal mechanisms the development of motor responses will be examined in several respiratory muscles that discharge in phase with expiration. Then, using vagal cooling as well as direct recording from vagal afferent units, the development of afferent vagal activity will be compared with the development of expiratory motor responses. This animal model may provide a basis for understanding the mechanisms underlying the maturation of neural pathways promoting active expiration.